In a disk drive, a magnetic recording head is made of read and write elements. The write element is used to record and erase data bits arranged in circular tracks on the disk while the read element plays back a recorded magnetic signal. The magnetic recording head is mounted on a slider which is connected to a suspension arm, the suspension arm urging the slider toward a magnetic storage disk. When the disk is rotated the slider flies above the surface of the disk on a cushion of air which is generated by the rotating disk.
The read element is generally made of a small stripe of multilayer magnetic thin films which have either magnetoresistance (MR) effect or giant magnetoresistance (GMR) effect, namely which changes resistance in response to a magnetic field change such as magnetic flux incursions (bits) from magnetic storage disk. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance in the read element and a corresponding change in the sensed current or voltage.
FIGS. 1 and 2A–2C illustrate examples of a conventional composite type thin-film magnetic head 10. FIG. 1 is a cross-sectional view of the head 10 perpendicular to the plane of the ABS. FIG. 2A shows the slider 11 flying above the disk 13.
In these figures, the reference numeral 12 denotes a substrate, 15 denotes an undercoating, 20 denotes a lower shield layer of the MR reproducing head part, 21 denotes an upper shield layer of the MR head part, which also acts as a lower pole of an inductive recording head part, 22 denotes a MR layer provided through an insulating layer 23 between the lower shield layer 20 and the upper shield layer 21, 26 denotes a recording gap layer, 27 denotes a lower insulating layer deposited on the upper shield layer 21, 28 denotes a coil conductor formed on the lower insulating layer 27, 29 denotes an upper insulating layer deposited so as to cover the coil conductor 28, 30 denotes an upper pole, and 34 denotes a pad positioned directly on the undercoating 15. Note that the pad 34 connects directly to the coil conductor 28. The upper pole 30 is magnetically connected with the lower pole (upper shield layer) 21 at its rear portion so as to constitute a magnetic yoke together with the lower pole 21.
As recording density and data transfer rate are increased 100% and 50% respectively for the past a few years, critical dimensions in recording device such as track width read and write gap and coil size have decreased accordingly. Also, the flight height between the air bearing surface (ABS) 32 and the media have become smaller and smaller. For reference, recording heads with 40 gb/in2 products typically have fly heights of 6–12 nanometer. This reduction in head critical dimensions and fly height, while beneficial to magnetic performance, also comes with cost on thermal and mechanic reliability.
The thermal expansion coefficients for the substrate and the various layers of the head differ, so when the head becomes heated during use, some layers will begin to protrude from the ABS. FIG. 2B depicts the head 10 when the write element is operating with current passing through the coil, and particularly illustrating protrusion of the layers that occurs during use. FIG. 2C is a detailed diagram of the heat transfer and protrusion profile of the head 10. One particular problem appearing in the latest generation of heads is that the write-induced protrusion of the pole and overcoat can cause head-media contact, resulting in hard disk drive failure. In older generations of heads, this was not a problem because the head was flying much higher and device size was bigger leading to easier heat dissipation. However, the latest generation of heads fly 60–120 Å above the media and the flight height is projected to decrease further for future products. Further, the coil length in modern heads has decreased to accommodate high data rate advancement. Consequently, ohmic heating from write current through coil and eddy current in write pole/yoke and magnetic hysteresis of magnetic materials are confined in a tiny space near ABS, which leads to unacceptable thermal protrusion and drive reliability failures. As can be seen in FIG. 2B, the top write pole 30 and overcoat protrude from the ABS 32 toward the media 13. The protrusion amount can be as high as 5–7 nanometers.
The thermal expansion is proportional to the temperature, so it would be desirable to reduce the temperature in order to limit the thermal expansion. This in turn would reduce protrusion.
The undercoating 15 in standard heads is a poor thermal conductor, and therefore effectively blocks heat transfer from the write element to the substrate 10, which could otherwise act as a heat sink. Undercoating materials used in magnetic recording heads are typically made of sputtered amorphous Al2O3 with a thickness ranging from 2 to 5 microns. As amorphous Al2O3 is a very poor thermal conductor. The high thickness of the undercoating 15 has further compounded the problem of poor heat dissipation from write element to substrate. The current method for planarizing the undercoating 15 is mechanical lapping to a target thickness. Drawbacks of lapping are high defect rate and poor thickness uniformity control. Thus, the undercoat thickness can be made no thinner than 2 micron.
The prior art did not recognized a solution to the problems appearing in the latest generation and future generation of heads, because, as mentioned above, protrusion did not present a significant problem in old system having higher fly height (150–500 A). In addition, the thick UC in earlier generations of recording heads was required due to the pad design in which the pad directly sits on the top of the UC surfaces. Noise from substrate would have been picked up if a thin UC had been used. It would be desirable to add, more planarization layers to recording heads (as shown in FIG. 4), such that the contact pads no longer sit directly on the top of UC.
A further problem is that the temperature rise reduces the life of the read sensor. The reader element is made of multilayer ultra thin layers and antiferromagnetic materials. High temperature causes interface mixing leading to low GMR coefficient and thus reduces the readback signal. In many hard disk drives, the read element remains in active mode during writing. A typical read element has a narrow stripe in the range 500–2000 Å and passes sensing current in the range of 2–6 mA. Such high current density will induce temperature rise in the read strip region to over 120–200 C in operating mode. Writing induced heat will further increase the read element temperature, resulting in either shortened life time of read element or will force the read element to work at a lower current, leading to a lower playback signal.
It would therefore be desirable to overcome the heretofore unaddressed problems appearing in the latest generation of heads, and sure to appear in future generations, one such problem being that thick undercoating materials coupled with the poor thermal conductivity of the industry standard amorphous Al2O3 render heating conduction from the writing element to the substrate body ineffective.